Discussion
Reductions in suspended sediment loss, relative to the experimental
control, were ordered: willow buffer strips (44%) >
deciduous woodland buffer strips (30%) > grass buffer
strips (29%). These results for the establishment phase of the buffer
treatments therefore suggest that woody treatments improve sediment
trapping the most. Clearly, these preliminary results might change as
the vegetation treatments mature over time. The zero efficacy in in the
first monitoring year across all buffer treatments and in the second
monitoring year for the willow and deciduous woodland buffers reflected
the soil disturbance associated with the establishment of the three
vegetation treatments. The impact of soil disturbance associated with
the installation of other sediment mitigation measures on farms, such as
channel bank reprofiling and fencing, and has been reported by other
studies (Lloyd et al., 2016).For the establishment period monitored in
this study, our results suggest that the grass buffer strip treatment
matures faster with respect to sediment trapping than the other two
woody vegetation treatments. Over time, the potential for buffer strip
saturation with trapped fine-grained sediment could be expected to
increase.
Direct comparisons of experimental results for buffer strip efficacy are
typically compromised by various factors. These include the contrasting
climate, soil types, runoff lengths, vegetation types and agricultural
practices of research sites. Additional potentially confounding factors
include deployment of different research infrastructure and study scales
and durations. Nonetheless, it is useful to interpret our new data on
buffer strip efficacy for reducing sediment loss in the context of
existing evidence. Working on 6 m buffer strips with fescue, shrubs and
trees, serving 3% slopes, Borin et al. (2005) reported a sediment
trapping efficacy of 93%. Schmitt et al. (1999), comparing 7.5m and 15
m grass, shrub and sorghum buffers, serving a slope of 6.7%, reported a
sediment trapping efficacy range of 63-93%. Syversen (1995), testing 3,
10 and 15 m grass and shrub buffer strips serving slopes of 14% and
28%, reported efficacies of 61-91%. Schwer and Clausen (1989) working
on 26 m wide grass buffer strips, serving slopes of 2%, reported a
sediment trapping efficacy of 95%. Across the existing scientific
literature reporting reductions in sediment loads due to buffer strips,
the efficacy range is typically 40% to 100% with reductions of
>50% commonplace (Dorioz et al., 2006). Given the close
functional relationship between fine-grained sediment and phosphorus,
efficacy ranges for reductions in particulate phosphorus loads are
similar. Our new results for reductions in sediment loss are reasonably
well aligned with, although slightly lower, than existing understanding
of reductions in sediment loads, although it is important to acknowledge
that our study represents the establishment phase of the vegetation
treatments. On that basis, the overall efficacies for the study period
should be viewed as being underestimates of longer-term performance.
Previous work has underscored the potential for reductions in sediment
loss to be strongly influenced by deposition of incoming sediment along
the upslope leading edge of buffer strips due to the initial reduction
in runoff velocity and sediment transport capacity (Ligdi and Morgan,
1995; Pearce et al., 1997). Such edge effects were not observed during
our experiment.
Excess sediment loss from agricultural land remains a global issue
despite the uptake of best management practices. For England and Wales,
for example, such elevated sediment losses due to current structural
land cover have been estimated to result in £523M of environmental
damage costs annually, with the uptake of best management practices on
farms only reducing those societal costs to £462M (Collins and Zhang,
2016). Buffer strips continue to feature in the mix of best management
practices implemented on farms to protect water quality and their uptake
by farmers can be facilitated by robust evidence on the efficacy for
reducing water pollution. Agricultural runoff encountering a buffer
strip meets a more porous and rougher surface, resulting in a reduction
in runoff velocity and sediment transport capacity. Here, the vegetation
cover generates increased resistance to runoff and sediment transport
and the root systems increase the permeability of the soil surface,
thereby encouraging infiltration and deposition (Magette et al., 1989;
Rose et al., 2003).
Buffer strips can also assist in the management of the sediment problem
by stabilising and reducing the erosion of riverbanks (Kemper et al.,
1992; Bowie, 1995) and by displacing sediment generating land management
away from watercourses (Wenger, 1999). The beneficial effects of
displacement are often, however, less pronounced on heavy meandering
watercourses where channel migration drives bank erosion (Williamson et
al., 1992). In England Wales, eroding channel banks have been estimated
to contribute 22% of the total fine-grained sediment load delivered
rivers and streams (Zhang et al., 2014). The potential beneficial
impacts of buffer strips on reducing bank erosion across England and
Wales, as well as sediment loss from utilised agricultural land, should
therefore be borne in mind given the important role of bank erosion in
the excess sediment problem nationally.
When interpreting evidence for buffer strip impacts on sediment loss, it
is important to acknowledge various issues which can confound efficacy.
Buffer strips can be prone to silting up, especially when soils are
saturated (Barfield et al. 1979; Hayes et al., 1979). Under such
conditions, deposited sediment is likely to remain unconsolidated and
prone to remobilisation, especially when a sequence of extreme storm
events occurs or buffer strips are breached by concentrated runoff in
preferential flow paths. Sediment trapping by buffer strips is commonly
particle size selective with coarser particles preferentially retained
(Hayes et al., 1984; Robinson et al., 1996; Hickey and Doran, 2004).
Here, particle size selectivity is often buffer width dependent, with
narrow 1 m buffer strips only trapping the coarsest particles (Hayes et
al., 1979). Vegetation management can influence buffer strip efficacy
for reducing incoming sediment loads since, for example, long grass is
more prone to lodging, which can permit preferential flow routes and
reduced efficacy. Incoming flow mechanisms can influence efficacy for
reducing sediment loads with, for example, concentrated flows reducing
efficacy (Dillaha et al., 1986; Dosskey et al., 2002; Canqui et al.,
2004). At our experimental site, however, pervasive raindrop-impacted
saturation-excess overland flow has been identified as a primary
mechanism for sediment mobilisation and delivery, rather than
concentrated runoff (Pulley and Collins, 2019). Finally, in real-world
settings, buffer strips serving agricultural land can be bypassed by
field drains (Haycock and Muscutt, 1995; McKergow et al., 2003), meaning
that the reductions in sediment loads relate to the surface runoff
pathway. In England and Wales, a considerable proportion of farmed land
has assisted underdrainage in support of productive agriculture
(Robinson and Armstrong, 1988), and field drains represent an important
sediment delivery pathway (Deasy et al.,2009; Zhang et al., 2016). This
means that more engineered buffer strip solutions will be required to
deliver multi-pathway control of sediment pollution in many parts of
England and Wales. Such solutions might, for instance, include the
cutting back of field drains to permit the construction of artificial
wetlands (Lenhart et al., 2016) thereby delivering a ‘treatment-train’
strategy combining buffer strips and wetlands. Where woody vegetation on
buffer strips is harvested, the timing of such management activities
will be critical to minimise compaction issues since these could reduce
sediment trapping efficacy.